--> Hydro-Mechanical Behaviour Of Porous Carbonate Rocks Across The Brittle-Ductile Transition Monitored By Ultrasonic Wave Velocities

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Hydro-Mechanical Behaviour Of Porous Carbonate Rocks Across The Brittle-Ductile Transition Monitored By Ultrasonic Wave Velocities

Abstract

Compactive and dilatant deformation in porous rocks is a crucial problem in fault development, geotechnical engineering and reservoir management. Active tectonics and extraction of fluids from buried porous host rocks modify the pore pressure in a reservoir, causing variations of the effective stress possibly leading to faulting and inelastic deformation that can in turn adversely affect oil production. Moreover, the mechanical and physical properties of reservoir rocks are affected by the nature of the fluids saturating their pore space and it has been shown that saturating the reservoir with water can result in a reduction in rock strength leading to enhanced compaction which in turn can impact reservoir productivity. Here we study the hydro-mechanical behaviour of synthetic porous carbonate rocks tested under a range of stress conditions in dry and water saturated states and monitor the evolution of their elastic properties using ultrasonic wave velocities.

Blocks of synthetic limestone were fabricated using the Calcite In situ Precipitation System (CIPS), from two initial powder mixtures: • Type A: 95 % calcite and 5 % quartz; • Type B: 45% calcite, 36% dolomite and 19% quartz.

a proprietary mineral cementation grouting technology was used to consolidate the powders. Following the initial petrophysical characterization, high pressure geomechanical tests were conducted to characterize the behaviour of the samples under dry and water saturated conditions while monitoring elastic wave velocities at ultrasonic frequencies. Three types of geomechanical tests were performed covering the brittle to ductile range of rock responses: i) unconfined compressive strength (UCS); ii) multistage triaxial (MTXL); and iii) hydrostatic (isotropic) compaction. Experimental results show that in all tested stress configurations water-saturated CIPS-cemented samples are weaker and more compliant than the dry equivalent. The response of the porous samples to the applied stress results in a modification of its pore space such that the rock may compact or dilate. The inelastic deformation of the samples undergoes a transition from brittle faulting to ductile flow with increasing mean effective stress (P) and can be illustrated by scaling the experimentally determined yield points by the critical pressure P* required for pore collapse.

At relatively low confinement (P/P* < 0.25), the inelastic deformation is dilatant and results in a slight increment of the initial permeability; under these conditions the sample would fail by developing a discrete failure plane oriented at high angle to the imposed maximum principal stress. At higher confinement (0.25 < P/P* 0.6), inelastic deformation is compactant and results in the development of shear enhanced compaction bands i.e. localised zones of intense cataclastic deformation oriented at low angle to the maximum principal stress that tend to decrease the permeability of the samples. At confinement P/P* > 0.6 deformation is still compactant and results in discrete compaction bands oriented normal to the maximum principal stress. For P/P* approaching 1 deformation occurs by a significant volume decrease even in the absence of any deviatoric stress via homogeneous cataclastic flow with associated considerable reduction in sample permeability.

The transition from brittle faulting to calaclastic flow is related to the initial rock porosity and its evolution with stress: the evolution can be dilatant or compactant. Whereas dilatancy is observed as a precursor to brittle faulting, failure by cataclastic flow may be accompanied by a positive or negative change in volume depending on the trade-off between pore collapse and microcracking. Moreover, since calcite requires relatively low shear stresses to initiate mechanical twinning and dislocation, these latter mechanisms play also a significant role in particular at high levels of plastic strain. The complex interplay between pore collapse, microcracking and crystal plasticity hence controls the carbonates macroscopic failure behaviour.